When a patient is suspected of having a tumor, a piece of tissue from the suspected site is removed surgically by a physician and is sent to an anatomic pathology laboratory for examination. Typically, the suspected tissue is fixed in formalin, processed in graded alcohols and xyline, embedded in paraffin, sliced into sections with a microtome and stained with hematoxylin-eosin (H&E). The pathologist then examines the stained tissue slides microscopically. Based on the distinct features of the parenchymal and stromal cells in the tissue, the pathologist then makes a diagnosis.
The H&E technique has proven to be one of the most durable in medicine and has remained essentially unchanged for over half a century. It is relatively quick, inexpensive, and suitable for most situations. Most important, it allows an accurate microscopic diagnosis of a large majority of sampled tissues. However, H&E staining has limited applicability in certain situations. For example, H&E staining is insufficient for etiologic, histogenetic or pathogenetic inquiries. Using the H&E technique, a pathologist can usually make a diagnosis of malignancy or benignicity. However, it is difficult, if not impossible, to identify the line of differentiation of the tumor. As a consequence, pathologists have always searched for additional techniques to probe those questions because identification of the line of differentiation of the tumor is crucial for the tumor's management. For instance, a poorly differentiated malignancy with B lymphocytic differentiation and CD20 positivity could be curable with RITUXAN™.
Another technique used by pathologists is immunohistochemistry (IHC). This technique was introduced 50 years ago for detection of antigens in frozen tissue with immunofluorescence. The science of antigen detection with visibly tagged antibodies did not come to life until the advent of monoclonal antibodies in the 1980s. In combination with an improved detection system, antigen epitope retrieval and automation, IHC is now an essential part of an anatomic pathology laboratory. It can be used to detect virtually any immunogenic molecule with remarkable sensitivity and specificity. It can also be evaluated against the morphologic backgrounds with which pathologists have long been familiar. With current IHC techniques, approximately 90% of the tumors with diagnostic difficulties by H&E technique can be accurately classified.
One of the crucial aspects of IHC is to have tumor-specific, tissue-specific or cell lineage-specific antibodies. Although all of our tissues and cells come from a single stem cell, during tissue differentiation different tissues and cells may have different antigenic profiles. For instance, thyroid transcription factor (TTF-1) is a nuclear protein transcription factor selectively expressed during embryogenesis in the thyroid, the diencephalon of the brain and respiratory epithelium. It is found only in thyroid tumors regardless of histologic types, as well as in lung carcinoma. The utility of TTF-1 becomes readily apparent in the differential diagnosis of primary versus metastatic carcinomas, especially in the lung or in pleural effusions.